1. Field of the Invention
The present invention has for its object to provide a control circuit for amplified listening comprising a shunt regulator arranged for regulating at a first reference value the supply voltage of an amplifier whose output is connected to a listening device, as well as a control device producing a control voltage for influencing the gain of the amplifier at least as a function of the current passing through the shunt regulator and including a storage device for storing a gain control voltage.
2. Description of the Related Art
Such a current is known from European Patent Application 189 711. With this circuit, the gain of the amplifier is reduced when the current passing through the shunt regulator decreases significantly. When the current passing through the shunt regulator is considered normal, the gain of the amplifier retains its nominal value.
This circuit is specifically intended to operate if there are signals having a constant amplitude (for example, dialling tones). On the other hand, it cannot be used to correct the gain in the presence of signals whose amplitude varies constantly (speech) because, without any time delay, it thus acts as a simple peak limiting circuit producing harmonics and thus distortion. In addition, it consumes a maximum current when the current available on the line is minimal.
The circuit as defined in Application document TEA 7531 by THOMPSON-CSF implements the same basic principle but with a considerable modification consisting in using a parallel resistance-capacitance circuit in which the gain of the amplifier is controlled as a function of the value of the voltage at the terminals of said capacitance. This arrangement has the disadvantage that it is intrinsically unstable and can only operate in an acceptable manner with long time-constants (various seconds) which are liable to reduce the amplitude of the oscillations.